Self-Catalyzed Destruction of Cytochrome P-450: Covalent Binding of Ethynyl Sterols to Prosthetic Heme

• Published in: Proceedings of the National Academy of Sciences - PNAS 1979-02, Vol.76 (2), p.746-749
• Main Authors: Paul R. Ortiz De Montellano, Kunze, Kent L., Yost, Garold S., Mico, Bruce A.
• Format: Article
• Language: English
• Subjects: Absorption spectra; Acetates; Adducts; Animals; Cytochrome P-450 Enzyme System - metabolism; Cytochromes; Liver; Liver - drug effects; Liver - metabolism; Male; Microsomes, Liver - metabolism; Norethindrone - pharmacology; Pigments; Pigments, Biological - metabolism; Porphyrins; Prosthetics; Rats; Sterols; Zinc
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.76.2.746

The hepatic pigment accumulated as a consequence of the self-catalyzed destruction of cytochrome P-450 by norethisterone (17-hydroxy-19-nor-17α -pregn-4-en-20-yn-3-one), after acidic methylation and purification, consists of two virtually identical, probably isomeric, porphyrins. Radiolabeled norethisterone is incorporated into both porphyrin products. The major of the two porphyrins exhibits a mass spectrometric molecular ion exactly equivalent to the sum of norethisterone and dimethylprotoporphyrin IX, less two hydrogen atoms: unequivocably demonstrating covalent association of the sterol with this porphyrin in a 1:1 ratio. Cytochrome P-450 is therefore destroyed by self-catalyzed addition of norethisterone to its heme prosthetic group. Cytochrome P-450 is also destroyed by norgestrel (13-ethyl-17-hydroxyl-18,19-dinor-17α -pregn-4-en-20-yn-3-one) and by 1-ethynylcyclohexanol but not by 17-hydroxy-19-nor-17α -pregn-4,20-dien-3-one. The destructive potential is thus clearly a property of the propargylic alcohol function. A mechanism involving enzymatic oxidation of the triple bond is postulated.